The invention relates to a process and device for contact-free thickness measurement in which a light source produces a sharply bundled light ray, a light deflector and ray divider are provided so that at least two simultaneously and periodically deflectable light rays are generated, and where devices are provided with which one of these light rays can be projected on one surface of the object and another of these light rays on the opposite surface of the object. Furthermore, photodetectors are provided which can pick up light only along a predetermined sighting line. With these detectors, time points can be determined at which points on the object surfaces lying on the respective sighting lines are hit by light rays.
From U.S. Pat. No. 4,068,955, a corresponding process is known in which the thickness of a workpiece with plant parallel surfaces is measured in a contact-free way. To that end, laser beams are directed step-wise over the opposite surfaces of the workpiece, and are diffusely reflected therefrom. The reflected rays are then registered by detectors which can only record light from a certain direction. The positions of the surface points at which the reflected light rays are registered by the detectors are determined from the time differential between the beginning of the light ray deflection and the reception of a light signal in the corresponding detector. If several points are scanned on the workpiece surface, then the workpiece may be tilted with respect to a virtual plane of referece during the measurement. In this case it is possible to determine the thickness of the workpiece independent of the tilting.
According to the aforementioned application the laser beam is split up into two partial rays. In the process the one partial ray serves for scanning the top side of the workpiece and the other partial ray serves for scanning the underside of the workpiece. In order to respectively positionally determine several points on the top or bottom side of the workpiece, these partial rays are split up into additional partial rays by prisms.